There exits a technology called “super-resolution processing/super-resolution conversion” that improves a resolution by using multiple frames of image data in a certain resolution.
The use of this technology enables conversion from a low resolution image to a high resolution image, and the high resolution image can be obtained with the same device as a conventional one (“Super-resolution processing With Multiple Digital Image Data” (Ricoh Technical Report No. 24, NOVEMBER, 1998)).
The super-resolution technology is widely applied in fields such as motion picture because multiple frames of image data having minutely different reading positions of an original document image in terms of subpixels (a unit smaller than one pixel) of the original document image are necessary to perform the super-resolution technology.
However, to perform the super-resolution processing, there exists a problem that the amount of data and the amount of calculations become large because the multiple frames of image data are necessary to generate one pixel of a high resolution image.
To this end, the amount of calculations has conventionally been reduced by determining the number of frames subjected to the super-resolution processing according to the size of an image area of interest (Japanese Patent Application Laid-Open No. 2006-092450).
However, in the conventional technology as described above, the number of images subjected to the super-resolution processing is determined only in the area of interest. Thus, it is necessary to previously ensure, in the entire image area, the number of frame image data needed for the super-resolution processing.
In a case where the super-resolution processing is used for a multifunction apparatus, or an MFP (Multifunction Peripheral), namely, image processing apparatuses, a line sensor is generally used in a reader in the multifunction apparatus, a scanner, and the like.
That is, the number of read frames obtained through one reading operation is one frame.
The above-described reader reads an original document image using a group of pixel sensors arranged horizontally spaced apart by a distance of integral multiples of a pixel in a primary scan direction. At this position being read, it is impossible to read the original document image with a minute (subpixel) shift in the primary scan direction.
To this end, an area sensor is inclinedly arranged in the apparatus, so that one reading operation allows obtaining image data with the minute shift in the primary scan direction and/or a secondary scan direction at the position of the pixels being read.
However, at this occasion, multiple frames of low resolution frame image data having been read are used regardless of output conditions of the image data.
Thus, there exists a problem that the amount of data and the amount of calculations increase because the low resolution frame image data are memorized more than necessary and that the super-resolution processing is performed using them.